1. Field of the Invention
This invention relates in general to shaft seals of the dual lip radial type and in particular to a shaft seal having a first annular sealing disk engageable with a shaft surface for sealing and retaining lubricant and a second annular sealing disk for preventing dirt and other contaminants from migrating along the shaft surface to the first sealing disk and into the lubricant.
2. Description of Prior Developments
Dual lip radial seals are well known in the art. In one known arrangement, such a seal includes an outer metal case having a cylindrical wall and a radial wall. An annular gasket is seated on the radial wall for engagement with a dirt excluder lip or disk. A second seal disk or lip is clamped against the dirt excluder disk by an inner metal case that is nested within the outer metal case. The inner metal case has a radial wall for applying a squeezing pressure on the second seal disk such that the two disks are mechanically retained in the metal case structure.
The two disks may be formed of polytetrafluoroethylene (PTFE). An advantage of this design is that one disk may be filled with one type of filler material such as glass for abrasion resistance, and the other disk may be filled with another type of filler material such as molybdenum disulphide for reducing running friction. A drawback associated with this design is its relatively high cost due to its numerous components and involved manufacturing method.
In another known arrangement, a relatively thick PTFE disk is bonded to an inwardly radiating wall portion of a metal case. The relatively thick disk has a radial slit extending from its inner edge partway toward its outer edge, such that two annular sealing disks or lips are formed by a single slit disk. The sealing disks are arranged in opposite axial directions on a shaft surface, whereby one of the sealing disks constitutes a dirt excluder seal and the other disk constitutes an oil seal. An advantage of this design is its low cost due to its simplicity of design and ease of manufacture. However, it is most difficult to provide separate fillers in the two seal lips in an economical fashion.
Accordingly, a need exists for a dual lip seal which incorporates the low cost advantage of the bonded lip design of FIG. 2 with the material and filler selectivity of the clamped lip design of FIG. 1. As can be seen in FIG. 2, two separate seal disks cannot be bonded to the radial flange 15 because once the first disk is bonded to flange 15 the first disk covers the metal bonding surface thereby preventing the bonding of a second disk to flange 15.